My feeling about the DC reboot, and this is just an opinion based on the comics I've read and not really backed up with facts, is that it was done in order to make the comics more palatable to movie makers. The publishers can simply hand a New 52 trade to a studio executive and they'll have an updated piece of source material to hand off to writers and directors.
I have a similar hunch about this Age of Ultron story. Age of Ultron involves Hank Pym and time travel. There's an Ant Man movie coming up. Marvel is not afraid to alter its own history to suit its needs (i.e. One More Day). If someone interested in the Ant Man movie looks up the character on Wikipedia, they'll find out that Pym beat his wife, had several psychological breakdowns and created a genocidal robot. This might turn away people interested in the movie.
Age of Ultron can give Marvel a way to change all of that, and make the character more appealing to the casual movie viewer, just in time for the Ant Man movie to come out. That Wikipedia page can then be changed to reflect a new, streamlined history. And movie ticket sales don't have to suffer because of some "silly comicbook stories" (ugh).
Or maybe I'm just getting too cynical in my old age.